Composites of steel and rubber wherein reinforcing elements of steel or steel alloys are bonded to rubber find wide application in a variety of products which include, for example, conveyor belts, heavy duty hoses, pulley belts and pneumatic tires. The application of such composites in the fabrication of pneumatic tires has become particularly important. That is, there has been an ever increasing use of steel cord to provide the need for more effective reinforcement in modern vehicular tires.
The traditional method for bonding rubber to steel is to plate the steel surface with brass for direct contact to rubber. However, because of its unpredictability, sensitivity to processing conditions and rubber composition together with a tendency towards rapid deterioration in chemical environments, new approaches to bonding these materials have been sought for some time as a replacement for the brass plate method. In seeking a new approach particular attention has been given to the use of various organic resin adhesives for application to the steel surface prior to pressing into contact with the rubber component. During the course of such investigations, it has been found that the well known RFL type organic adhesives, which have been used for many years in the bonding of polymeric fibers to rubber, offer exceptional promise as a practical means for bonding steel to rubber without the problems associated with brass plating.
The basic RFL adhesive composition was first described in U.S. Pat. No. 2,128,229 which issued in 1938 and consists of an aqueous emulsion of a resorcinol-formaldehyde resin and a rubber latex. Although there have been various improvements and additions to this basic formulation over the years, such as are described in U.S. Pat. Nos. 3,817,778 and 3,835,082, these additions and improvements have not fundamentally altered the character of the basic composition and all such variants are generally referred to as RFL type organic adhesives.
Although the use of these adhesives as the bonding agent in the fabrication of steel and rubber composites has produced generally satisfactory results, a drawback has recently been observed which is manifested when articles comprised of such composites are subjected to moisture-containing environments during use. That is, it has been found that the bond between the adhesive and the steel surface becomes weakened in the presence of water. When exposure to moisture is prolonged, the steel reinforcing element tends to separate from the adhesive binding it to the rubber and the separated surface becomes exposed to the corrosive effects of the aqueous environment. Eventually, the deterioration caused by this separation and corrosion results in failure of the composite product.
This problem was first observed during the course of a test program on steel radial tires in which an RFL adhesive was used to bond the steel to rubber. The test tires were installed on a fleet of vehicles which were driven in areas of rough terrain and often under wet conditions. As a result, the tires incurred various cuts or punctures while under test in this environment. It was noticed that after the appearance of these punctures or cuts (say between about 5,000 and 27,000 miles later depending on number or size) a debonding between the steel cords and the rubber took place. The debonding manifests itself in the early stages by the movement ("wire walking") of steel cord toward edges of the belt. Eventually the cords penetrate the tire tread at the belt-sidewall interface and bare cord is observed protruding from these areas. This phenomenon is called whiskering. Complete separation between the belt and tread finally occurs in the later stages with considerable cord corrosion being observed.
This loss of adhesion between steel cords and rubber when tires are cut (or punctured) and driven under wet conditions is attributed to a weakening of the adhesive bond between the RFL adhesive and the steel surface when exposed to moisture. The moisture, of course, gains entrance to the bonding system via cuts or punctures in the tire.
It is, therefore, an object of this invention to improve the hydrolytic stability of the adhesive bond between an RFL type organic adhesive and the surface of steel reinforcing elements employed in composites of rubber and steel.
It is a further object of the invention to prevent the loss of adhesion between steel reinforcing elements and rubber when vehicular tires become cut or punctured and are driven under wet conditions.